Situations arise in which it would be convenient to have a distributed means of providing heat in circumstances where heating appliances are not available. For example, producers of prepared foods have indicated that there could be significant market potential for self-heating food packaging (SHFP) systems that could heat prepared foods in their containers to serving temperature, simply, safely, and efficiently.
For a mass consumer SHFP product, safety is paramount and should be inherent; preferably there should be no exposure of users to extreme temperatures, no fire, and no smoke or fumes under anticipated use and abuse conditions. Practical considerations mandate that any system be reasonably compact and lightweight with respect to the food to be heated. Thus, the system should have a good specific energy and high thermal efficiency. The system must also be capable of extended storage without significant loss of function or accidental activation of the heater. There should be some simple means of activating the heater, after which the required heat load should be delivered efficiently within a specified time period or about one to four minutes. Operation must be very reliable with low failure rates in millions of units of production. For a single use food application, material components should be food-safe, low-cost, environmentally friendly and recyclable.
The only SHFP technology currently in the general consumer market uses an onboard system for mixing separated compartments of quicklime and water, yielding an exothermic heat of solution. These products are bulky (literally doubling package size and weight), complex, unreliable, costly, and have achieved very low market penetration. There have also been reported instances of the heater solution leaking and coming into contact with food or consumers.
An exothermic reaction in which the component reactants could be premixed yet be inert until such time as the user initiates the reaction would be beneficial in terms of providing for a simpler, more compact, and low cost package design. A solid state reaction system could offer advantage over wet chemical systems since solid systems will be less prone to spill or leak.
While various solid state reactions can be considered, one exemplary solid state reaction is appropriately moderated thermite reactions. Thermites are a class of exothermic solid-state reactions in which a metal fuel reacts with an oxide to form the more thermodynamically stable metal oxide and the elemental form of the original oxide. Thermites are formulated as a mechanical mix of the reactant powders in the desired stoichiometric ratio. The powders may be compressed into a unitary mass. These compact reactions generate substantial thermal energy. However, thermite reactions typically require high activation energy and thus thermite reagent compositions can be formulated to be quite stable against inadvertent initiation due to electrostatic shock or mechanical impact. This generally inert character is an advantage in storage and transportation. Solid state thermite reaction formulations may also be formulated to yield precisely moderated reaction characteristics with a controlled solid flame front speed of less than 1 mm per second. Such moderated thermite formulations have negligible gas reaction products and could be readily integrated into heating device to achieve safe and efficient heating of the contents of a container within about one to four minutes.
Given certain preferred characteristics, other non-thermite kinetically moderated solid state reaction systems, such as moderated reaction compositions of iron powder fuel mixed with a strong oxidizer, are also suitable for self-heating applications. Preferred reaction systems would be comprised of premixed solid state reactants with high heats of reaction so as to yield compact high energy content devices that are inert and stable until deliberately activated.
Further, although once activated the energy-releasing chemical reaction may produce reaction intermediates in gas or liquid form, it would be preferable that the principal final products of the solid state reaction composition be solid materials, so that there is not undesirable volume expansion or pressure generation. Such solid state reaction systems, which would generate negligible gas reaction products, would also be amenable to being hermetically sealed into heating devices so as to fully contain any emissions, smoke, or odors that do occur, if a facile means of activating the sealed heating device can be provided.
The heaters that incorporate the solid state reaction system should be easily integrated into heating devices that provide for thermal product safety under anticipated use and inadvertent misuse by consumers.